Conventionally, in order to divide a wafer, on the surface of which semiconductor devices, electronic components and the like are formed, into separate chips, there has been used a dicing apparatus which forms grinding grooves in the wafer by a thin grindstone referred to as a dicing blade and cuts the wafer. The dicing blade is formed by making fine diamond abrasive grains electrodeposited by use of nickel, and the dicing blade having an extremely thin thickness of about 30 μm is used.
In the dicing apparatus, the dicing blade is rotated at a high speed of 30,000 to 60,000 rpm to form a cut in the wafer, so that the wafer is completely cut (full cutting) or incompletely cut (half cutting or semi-cutting). The half cutting is a processing method for cutting the wafer to a depth of about half the thickness of the wafer, and the semi-full cutting is a method for forming grinding grooves in the wafer by leaving about 10 μm of the thickness of the wafer.
However, when the grinding work is performed by the dicing blade, since the wafer is a highly brittle material, the work has to be a brittle mode and chipping is generated on the front surface and the rear surface of the wafer. This chipping causes the performance of divided chips to be degraded. In particular, the chipping generated on the rear surface makes a crack proceed into the inside of the chips, which is a troublesome problem.
Instead of cutting by use of the conventional dicing blade, as a method to solve the chipping problem in the dicing process, there is proposed a technique relating to a laser processing method in which laser light with a condensing point arranged inside the wafer is made incident so as to form a modified region by multi-photon absorption inside the wafer, and in which the wafer is divided into individual chips by using the modified region as a reference point (see for example, Japanese Patent Application Laid-Open No. 2002-192367, Japanese Patent Application Laid-Open No. 2002-192368, Japanese Patent Application Laid-Open No. 2002-192369, Japanese Patent Application Laid-Open No. 2002-192370, Japanese Patent Application Laid-Open No. 2002-192371, and Japanese Patent Application Laid-Open No. 2002-205180).
In this laser processing method, a modified region formed inside the wafer is formed at a constant depth from the surface of the wafer, so that it is necessary to detect the height of the surface of the wafer to control the position of the condensing point of the laser light.
For this reason, conventionally as shown in FIG. 5, laser light L1 emitted from a laser oscillator is made incident through the surface of the wafer W, and the condensing point is adjusted to the inside of the wafer, so as to make a modified region P formed inside the wafer, while laser light L2 partially reflected from the surface of the wafer is taken into an optical measuring device 129 through a half mirror 126 so that the position of the surface of the wafer is detected.
However, in the case of the method for detecting the surface of the wafer as shown in FIG. 5, the surface detection point and the processing point by the laser are the same. Thus, when the position of the condensing point of the laser light L1 for processing is controlled on the basis of detected position data in the thickness direction of the wafer surface, a delay is caused in the control because of the high processing rate.
As a result, there is a problem that when the laser light is scanned from the outside of the edge of the wafer toward the inside of the wafer, the condensing point of the laser light cannot be adjusted to the inside of the wafer in the vicinity of the edge of the wafer and thereby the modified region cannot be formed.
The present invention has been made in view of the above described circumstances. An object of the present invention is to provide a laser dicing apparatus in which when laser light is incident from the surface of the wafer and scanned, the position control of the condensing point of the laser light can be performed even at the periphery of the wafer, and in which a modified region by multi-photon absorption can be formed at a predetermined position inside the wafer.